Lapilli is a size classification of

tephra Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism. Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, they re ...

, which is material that falls out of the air during a

volcanic eruption Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are oft ...

or during some

meteorite impact An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or m ...

s. ''Lapilli'' (singular: ''lapillus'') is Latin for "little stones". By definition lapilli range from in diameter. A pyroclastic particle greater than 64 mm in diameter is known as a

volcanic bomb A volcanic bomb or lava bomb is a mass of partially molten rock (tephra) larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. Because volcanic bombs cool after they l ...

when molten, or a volcanic block when solid. Pyroclastic material with particles less than 2 mm in diameter is referred to as

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...

.How Volcanoes Work
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Formation

Lapilli are spheroid-, teardrop-, dumbbell- or button-shaped droplets of molten or semi-molten

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

ejected from a volcanic eruption that fall to earth while still at least partially molten. These granules are not accretionary, but instead the direct result of liquid rock cooling as it travels through the air. Lapilli

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

s are a very common form of volcanic rock typical of rhyolite,

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predo ...

and

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite ...

pyroclastic eruptions, where thick layers of lapilli can be deposited during a basal surge eruption. Most lapilli tuffs which remain in ancient terrains are formed by the accumulation and ''welding'' of semi-molten lapilli into what is known as a welded tuff. The heat of the newly deposited volcanic pile tends to cause the semi-molten material to flatten out and then become welded. Welded tuff textures are distinctive (termed ''

eutaxitic In igneous petrology, eutaxitic texture describes the layered or banded texture in some extrusive rock bodies. It is often caused by the compaction and flattening of glass shards and pumice Pumice (), called pumicite in its powdered or dust f ...

''), with flattened lapilli,

fiamme Fiamme are lens-shapes, usually millimetres to centimetres in size, seen on surfaces of some volcaniclastic rocks. They can occur in welded pyroclastic fall deposits and in ignimbrites, which are the deposits of pumiceous pyroclastic density cu ...

, blocks and bombs forming oblate to discus-shaped forms within layers. These rocks are quite indurated and tough, as opposed to non-welded lapilli tuffs, which are unconsolidated and easily

eroded Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is disti ...

Accretionary lapilli

Rounded balls of

are called ''accretionary lapilli'' if they consist of layered

particles. Accretionary lapilli are formed by a process of wet ash aggregation due to moisture in volcanic clouds that sticks the particles together, with the volcanic ash nucleating on some object and then accreting to it in layers before the accretionary lapillus falls from the cloud. Accretionary lapilli are like volcanic hailstones that form by the addition of concentric layers of moist ash around a central nucleus. This texture can be confused with spherulitic and

axiolitic A QAPF diagram is a double ternary diagram which is used to classify igneous rocks based on mineralogic composition. The acronym QAPF stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid (Foid)". These are the mineral groups used for ...

texture.

Armoured (or cored) lapilli

These lapilli are a variety of accretionary lapilli, though they contain lithic or crystal cores coated by rinds of coarse to fine ash. Armoured lapilli only form in hydroclastic eruptions, where significant moisture is present. The vapour column contains cohesive ash which sticks to particles within it.

References

External links

Volcanic Materials Identification

Tephra fall from Mt St. Helens
* {{cbignore (segment on accretionary lapilli beginning at 45:04 in video) Tephra Petrology Volcanic rocks

Formation

Accretionary lapilli

Armoured (or cored) lapilli

See also

References

External links